1. Field of the Invention
The invention relates to motor control apparatus and, more particularly, to a motor control apparatus for a railway switch machine having a reversible motor.
2. Background Information
In order to optionally switch a railroad train operating on a first track to a second, merging track, it is typical to provide a switch with a pair of "switch points" which are selectively movable horizontally to deflect the train toward one or the other of the tracks. The switch can encompass a pair of switch rail lengths of the second track which extend several feet in length with the switch points being essentially tapered end sections of those rail lengths. The switch points, typically labeled as "normal" and "reverse", are selectively movable back and forth between a pair of stock rails. These provide a normal position in which the train is directed toward the first track by the normal switch point being positioned against a first rail of the first track, and a reverse position in which the train is directed toward the second track by the reverse switch point being positioned against the opposite rail of the first track.
The switch points are typically attached to each other via a plurality of tie rods, at least one of which doubles as a switch throw rod. The throw rod is driven by a remotely controlled electrical switch machine, or, in some instances, by a hand lever operated switch machine, between extended and retracted positions. Depending upon the side of the track on which the switch machine is placed, the extended position can be the normal or the reverse condition of the switch points, and vice versa for the retracted position.
Switch machines employ reversible electric motors to drive a series of gears which are attached to the throw rod. Depending upon the control signals received at the switch machine, the motor is driven one direction or the other to either extend or retract the throw rod and, thus, move the switch points between normal and reverse switching positions. Lock connecting rods are also attached to the switch points. The lock connecting rods passively move back and forth with the switch points and cooperate with locking elements in the machine housing to lock the switch into a normal or a reverse switch position.
Referring to FIG. 1, a switch point adjuster 2 is schematically depicted. The exemplary switch point adjuster 2 utilizes two separate rods 3, 4 and a frog 5, although a single operating rod (not shown) may be employed. The exemplary switch point adjuster 2 is located at the center of the track 6, although other such adjusters may be employed on the left side (with respect to FIG. 1) and opposite the switch machine 8. The first rod 3 connects the switch point adjuster 2 to the frog 5, and the second rod 4 connects the switch point adjuster 2 to the operating bar 10 of the switch machine 8. Thus, when the switch machine 8 throws six inches, the slack is taken up in the switch point adjuster 2 so that the frog 5 is only moved its required amount. Both operating rods 3, 4 are supported by support rollers (not shown).
As shown in FIG. 2, a railroad switch includes a pair of switch points 12,14 which are linked by one or more tie rods 16. The switch points 12,14 are selectively movable between a "normal" position (as shown) and a "reverse" position. In the illustrated normal position, the switch point 12, commonly called the normal switch point, is positioned against a stationary stock left rail 18, and the switch point 14, commonly called a reverse switch point, is moved away from a stationary stock right rail 20. The stock left and right rails 18 and 20 are anchored to a plurality of cross ties 22 via rail anchors 24 in a conventional manner. In a normal position, the normal switch point 12 directs a train entering the railroad switch straight through the intersection via the right stock rail 20 and the switch point 12, which tapers outward into a straight left rail 26 past the switch.
In a reverse position (not shown) both the normal switch point 12 and the reverse switch point 14 are moved to the right (with respect to FIG. 2) with the normal switch point 12, thus, moving away from the stock left rail 18 and the reverse switch point 14 moving to a position against the stock right rail 20. The reverse switch point 14 is then in a position to direct the train to the left via the left rail 18, which curves to the left past the switch, and via the reverse switch point 14, which tapers outward to a curved right track 28 past the switch.
The switch points 12 and 14 are selectively moved via a switch machine 30. The switch machine 30 includes a reversible electric motor (M) 31 (shown in hidden line drawing) in a motor housing 32. The motor 31 is connected to drive a series of gears 33,34,35 (shown in hidden line drawing) which, in turn, drive a throw bar 36 (shown in hidden line drawing), either to the left or the right (with respect to FIG. 2). The throw bar 36 is connected to a throw rod 38 via a linkage 40. The throw rod 38, in turn, is connected to the tie rod 16 via a switch basket 41. The switch basket 41 is internally threaded to receive threads 42 on the throw rod 38, in order that the switch point position at either end of travel of the throw rod 38 is adjustable. For example, a typical stroke length for the throw bar 36 would be approximately five inches.
Historically, switch machine motor controls employed mostly 3-wire (FIG. 4) or 5-wire (FIG. 5) control for permanent magnet or wound field motors, respectively.
In a switch machine, linear motion of the mechanism moving the points (such as 12 and 14 of FIG. 2) is converted into rotary motion. Rotary operated cam switches, in turn, are used to open the motor circuit at the end of the stroke and steer current to change direction. FIG. 3 illustrates the operation of cam switch (CSw1) 44 and cam switch (CSw2) 46. For normal operation, CSw1 is closed for most of the cycle and, then, opens at the end of the cycle to open the motor circuit and stop the motor. For reverse operation, CSw2 is closed for most of the cycle and, then, opens at the end of the cycle to open the motor circuit and stop the motor.
FIG. 4 illustrates how the two cam switches 44,46, in conjunction with a three-pole switch (SW1) 48, are used to control a three-wire permanent magnet motor (M) 50 using 3 wires 52,54,56. In FIG. 4, the full normal rotation is shown completed, there is no normal (N) current, CSw1 is open and CSw2 is closed. When SW1 is moved from the normal (N) to the reverse (R) position, reverse (R) motor current flows as indicated and the cam revolves in a suitable rotational direction until CSw2 opens and the motor 50 stops.
FIG. 5 shows how two cam switches 58,60, in conjunction with a double-pole switch (SW2) 62, are used to control a wound field motor (M) 64 using 5 wires 66,68,70,72,74. In this case, motor armature current is reversed for each of the normal (N) and reverse (R) directions, but current through the field 76 is unidirectional. Operation of the cam switches 58,60 is the same as the respective cam switches 46,44 of FIG. 4.
For both FIGS. 4 and 5, the point at which the respective motors 50,64 stop is not well controlled. This is because the circuit is simply opened and the precise stopping point is determined by inertia and by friction that is to be overcome. The friction is not well controlled, particularly, because the switch machine must operate over a wide temperature range. It is common for the friction to be significantly greater in cold weather because grease at cold temperatures is more viscous.
U.S. Pat. No. 5,747,954 discloses a two-terminal configuration having contacts at the terminals of the motor. An electronic controller circuit for the power down function of a highway crossing guard mechanism eliminates the "pumping" condition that can cause undue stress and damage to the guard mechanism. For the up direction, power is supplied through one contact to the motor and through another contact from the motor. At approximately 90.degree., a controller contact opens and MCR contacts drop to their "b" positions. A contact feeds power to a hold clear solenoid coil that sets the brake for the gate arm. Then, for power down operation, power is supplied to the motor through a MOSFET and diode of a power down module until, at approximately 45.degree., a contact is opened. In many applications, it is desirable at one position to change from a motor-powered down mode to an electrical braking mode using the motor as a generator and employing dynamic braking of the crossing arm as it travels further downward to another position. A diode and a dynamic braking resistor in parallel with the motor, during movement between those positions, exercise some control over the speed of the descending gate arm.
U.S. Pat. No. 4,703,303 discloses a solid state railroad gate controller having a logic circuit.
U.S. Pat. No. 5,806,809 discloses a switch point detection system and method that uses a series of proximity detectors positioned proximate the switch point(s) of a railroad switch. A switch machine motor is connected by 3 wires to a biased neutral controller.
Although the prior art shows various types of motor controls for railway switch machines, there is room for improvement.